Fair value measurements Book and Fair Values (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	$ 3,256	$ 2,709
Book Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	3,256	2,709
Interest rate swap agreements	9,319	20,661
Secured notes payable	708,653	716,144
Unsecured senior notes payable	1,048,190	549,805
Unsecured senior line of credit	14,000	566,000
Unsecured senior bank term loans	1,100,000	1,350,000
Fair Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	3,256	2,709
Interest rate swap agreements	9,319	20,661
Secured notes payable	761,047	788,455
Unsecured senior notes payable	1,028,750	593,350
Unsecured senior line of credit	13,738	567,196
Unsecured senior bank term loans	$ 1,088,322	$ 1,405,124